The present invention relates generally to the field of semiconductor wafer testing probe systems and, more specifically, to a buckling beam probe system and to a process for manufacturing the system using insulated metal to hold the probe wires.
Small metal wires on the order of tens of microns (a few thousandths of an inch) in diameter are typically used to contact electrical test pads on individual die or to contact electrical test pads on portions of an entire semiconductor wafer. The configuration of the individual contact wires or probes depends on the layout of the test pads to be contacted. The test pads can be in a linear configuration, in an array, or in combinations. In most cases, the size of the test pads is on the order of 75 to 125 microns (three to five thousandths of an inch) in diameter and the center-to-center spacing from pad to pad can be as little as 85 microns (about three and one-half thousandths of an inch) for linear arrays and as little as 150 microns (six thousandths of an inch) for area arrays. The high density of electrical test pads on silicon die and wafers creates a challenge when attempting to contact the electrical test pads. Many different processes have been developed to meet that challenge.
One traditional process, directed to linear arrays, uses cantilever probes to contact the electrical test pads. These probes are small wires made from metal, having a high yield stress, such as beryllium copper or tungsten. The wires are formed in a long beam with a bend on one end that contacts the test pad. The other end of the wire is held by a card, usually made of polymer, that provides electrical interface to the test electronics and mechanical alignment and stability for the probe array. Typical industry examples are epoxy-ring and blade probe cards.
Area arrays present more of a challenge than linear arrays because the probe array must extend in a perpendicular direction away from the device under test (DUT) as opposed to a radial fan out direction for linear arrays. The density of an area array does not allow radial fan out of the probes. (xe2x80x9cFan outxe2x80x9d means that the probes have a greater pitch at their tips than at their bases.) Accordingly, vertical conducting wires are used to contact area arrays. The vertical probes must be held in place to maintain mechanical alignment and to ensure that the probe tips contact the test pads for the DUT.
Typically, polymer or ceramic materials are chosen to hold all of the vertical probe wires. Cobra(trademark) probes (available from International Business Machines Corporation of Armonk, N.Y.) and the buckling beam probes disclosed in U.S. Pat. No. 4,027,935 are both examples of vertical metal wire arrays held in place by polymer or ceramic materials. The probe wires can also be permanently bonded to a surface that is part of an electrical space transformer. The configuration of the bond pads on the space transformer is a mirror image of the configuration of the pads on the DUT. The probe wires are permanently bonded on the space transformer to form an array that will contact the array of pads on the DUT. In this case, the individual probe wires are held in place by a space transformer which functions as a three-dimensional fan out for the area array probesxe2x80x94thus bringing the high density of the probe array to a larger dimensional area array that is compatible with larger contact systems to interface with the test electronics. An industrial example of a bonded array probe is the system made by Formfactor, Inc. of Livermore, Calif. and disclosed in International Patent Application No. PCT/US97/08604.
The requirements for any probe system are electrical performance, mechanical and thermal stability, and manufacturability. Conventional Cobra(trademark) and buckling beam probes have good manufacturability because they can be repaired. These probes have poor electrical performance, however, because electrical coupling exists between the individual probe wires.
In contrast, bonded probes have good electrical performance because the probe wires can be shorter than Cobra(trademark) or buckling beam wires. But bonded probes have poor manufacturability: if one probe is damaged, it cannot be repaired. The entire system must be reworked because the probe system is integral with the space transformer.
The mechanical and thermal stability of either bonded probes or vertical wire probes can be adequate if the area of the probe array is not as large as an entire 200 mm (eight inch) wafer or greater. In this case, the thermal coefficient of expansion (TCE) becomes a major factor because many electrical tests are conducted at elevated temperatures. The space transformer holding the bonded probes or the materials holding the vertical wire probes must be matched to the TCE of the DUT which, in the case of silicon, is very low compared to most materials. Only certain ceramics and metals have a TCE matching silicon.
The requirements of probe systems for high-density chip and wafer testing must also be evaluated considering the future of device testing. The chip fabrication industry is moving in the direction of full wafer testing. This trend will require probe and space transformer systems that are suitable for large areas up to about 300 mm (twelve inches) in diameter. Although International Business Machines Corporation has developed and patented space transformers that satisfy this requirement, the present probe systems will not meet this requirement.
The buckling beam probe has been in use for over twenty years. It has high density and is both reliable and repairable. The probe has poor high-frequency characteristics, however, and is not suitable for large-area, elevated-temperature applications. One reason for these disadvantages is the polymer used to hold and guide the array of vertical probe wires. The dielectric properties of the polymer allow electrical coupling between the probe wires. The high inductance and AC coupling between adjacent probe wires render conventional probes suitable only for DC testing. This undesirable coupling is especially difficult because buckling beam wires are generally long to allow for buckling of the wires and permit compliance when contacting the DUT. The longer the probe wire, the worse the electrical coupling. In addition, the polymers used to hold the probe beams in place have a high TCE and are not thermally stable.
In view of the shortcomings of the prior art, it is an object of the present invention to provide a buckling beam probe system using insulated metal to hold the vertical beam probe wires. In this design, the desirable features of the buckling beam are preserved while correcting the deficiencies. The TCE of the metal can exactly match silicon (about 3 to 5xc3x9710xe2x88x926 per xc2x0C.). Because the material is metal, the beam probes cannot electrically couple. Another object of the present invention is to provide a buckling beam probe system in which the probe wires are not integral with the space transformer, thereby separating the probe system from the space transformer and facilitating repairs.
It is still another object of the present invention to provide a process for manufacturing a buckling beam probe system. The process makes each beam wire a transmission line with controlled impedance. Controlled impedance allows the beams to be as long as necessary for compliance with no degradation in high-frequency (above 300 mHz) electrical performance. It is a further object of the present invention to provide a buckling beam probe that allows high density (less than a 0.2 mm or an 8 mil pitch), high electrical performance, low inductance, and high reliability; is repairable and mechanically, dimensionally, and thermally stable; and provides testing compatible and consistent with the present and future needs of the chip testing industry. Such needs include testing of larger area arrays up to 300 mm (twelve inches).
To achieve these and other objects, and in view of its purposes, the present invention provides a buckling beam probe assembly for electrically connecting a test apparatus with contact pads on the surface of a device to be tested. The assembly comprises a plurality of buckling beam wires each having a head, a body, and a tail and being pressed vertically onto the contact pads and buckling laterally to adapt to height differences of the contact pads caused by irregularities on the surface of the device to be tested. A top plate has a first plurality of apertures receiving the heads of the plurality of buckling beam wires. A bottom plate has a second plurality of apertures receiving the tails of the plurality of buckling beam wires. A plurality of intermediate metal sections are positioned between the top plate and the bottom plate. Each of the intermediate metal sections have a plurality of openings, the openings being coated with an insulation layer and receiving the bodies of the plurality of buckling beam wires.
The process for making the buckling beam probe assembly according to the present invention includes the steps of:
(a) providing a top plate having a first plurality of apertures and a bottom plate having a second plurality of apertures;
(b) forming a plurality of intermediate metal sections each having a plurality of openings coated with an insulation layer;
(c) stacking the bottom plate and the top plate with the plurality of intermediate metal sections between the bottom plate and the top plate;
(d) loading a plurality of buckling beam wires into the apertures of the top and bottom plates and into the openings in each of the plurality of intermediate metal sections;
(e) shifting at least one of the plurality of intermediate metal sections with respect to the others of the plurality of intermediate metal sections; and
(f) applying fasteners holding in position the stack of bottom plate, shifted plurality of intermediate metal sections, and top plate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.